Every electrical device that connects with a power supply main is required not to pollute or transmit high frequency noise onto the main. The amount of electrical emission allowed by electrical devices is heavily regulated by the Federal Communications Commission (FCC). Conventional power supply designs have migrated to using higher operating frequencies since the higher operating frequencies allow a reduction in size of power supply components and allow a reduction is cost. The disadvantage of operating at higher frequencies is the increased production of high harmonics or electromagnetic interference (EMI).
Conventional methods of reducing EMI has been aimed at reducing a switching frequency of a switching circuit below standard EMI bandwidth limits of 150 Khz as set by the FCC. This approach has the disadvantage of increasing the size of magnetic components in a power supply. Other methods of reducing EMI have simply been to add additional filter components to reduce unwanted frequency harmonics. This second approach has the disadvantage of adding to the weigh, size and cost of power supplies. Another approach to reducing large spikes of harmonics or EMI is the use of a snubber. The snubber although effective in reducing EMI, compromises efficiency of a power converter. In yet another approach, EMI is reduced by using jitter that takes discrete harmonic spectrum and spreads the EMI over a continuous frequency range. Conventional systems use jitter by injecting noise into a gate drive or controller of the converter. Injecting noise into the gate drive of the converter has the disadvantage of distorting the output voltage signal. Further, injecting noise directly into a gate drive only applies jitter to the rising and falling edges of the converter switching signal. By injecting jitter directly into the gate drive circuit, conventional power converters inhibit the efficiency of feedback loop and other features including zero voltage switching and sampling of the switching signal.
A schematic diagram of a prior art regulated power supply 10 is shown in FIG. 1. The power supply 10 includes a converter circuit comprising a switch 14, a controller or gate drive 16 and a feedback circuit 18. The switch 14 is coupled with an output circuit comprising a rectifying diode D1 and an output capacitor C2. The power supply 10 includes a capacitor C1 and an inductor L1 coupled across an input voltage Vin. The gate drive 16 comprises a pulse width modulator (PWM) module that is coupled with a gate of the switch 14. The power supply 10 includes a voltage regulating circuit comprising the feedback circuit 18 and the gate drive 16. The gate drive 16 uses the PWM module to alter a duty cycle of the switch 14 depending on the feedback provided by the feedback circuit 18. The gate drive 16 accordingly adjusts the duty cycle of the switch 14 to compensate for any variances in an output voltage Vout. The power supply 10 limits EMI in the switch 14 by injecting jitter using a jitter mechanism within the gate drive 16. This jitter approach is plagued with the many disadvantages discuss above.
Accordingly, it is desirable to provide a regulated power supply with a jitter method and apparatus that is effective in substantially reducing EMI emission without the disadvantages of conventional systems.